iMac 2011 Thunderbolt not found?

Hello all! I have a 2011 iMac that I've loved for ages now, and it's been a perfect little champ to me. However, today I went to hook up my MacBook in target disk mode to it via Thunderbolt to do a couple large file transfers, and nothing happened. I checked that the port on my MB was working fine by hooking it to my mini server, and it worked fine. The thunderbolt port on my iMac seems completely dead - even when I look in System Profiler, it shows that there is no hardware found for thunderbolt. Any ideas what could have happened and if there's any way to fix this short of a new logic board? Thanks!

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